This invention relates to the field of solid state electronics and, more particularly, to a process for forming microcrystalline silicon material by bombarding a depositing film with inert gas ions.
Microcrystalline silicon material, defined as having a network of crystalline formations within an amorphous matrix, advantageous in many circumstances. For example, a large number of conductors make good ohmic contact to microcrystalline silicon, while only a few materials make good contact to amorphous silicon. The substitution of microcrystalline silicon increases the conductivity of a contact interface by approximately three orders of magnitude, making it possible to obtain good contact even with screen printed materials. The crystallites also increase doping efficiency of silicon over that obtainable in the amorphous state.
It has been determined that microcrystalline silicon, like amorphous silicon material, is very preparation dependent. Prior processes for forming microcrystalline silicon generally involve establishment of a glow discharge within a gaseous mixture of silane, hydrogen gas and a suitable dopant. The discharge is generally carried out at a pressure on the order of 0.06 torr and a power density between approximately 0.7 and 1.6 watts per square centimeter. The crystallinity of silicon deposited by gaseous discharge is known to increase with increasing power. However, the required power levels can have a deleterious effect on previously deposited layers of a device and can significantly increase processing costs. Processes of this type also require unduly long deposition times to obtain microcrystalline material.
A variety of diluents, including argon, have been used in the plasma deposition of silicon. However, as far as applicant is aware they have not been used for any operative effect or in a glow discharge atmosphere into which hydrogen gas is introduced as a constituent. In these systems, hydrogen gas is present, if at all, only by virtue of the dissociation and surface reaction of silane.
A system of the type described above, using argon as a diluent, is described in Matsuda, et al., "A Simplified Model for the Deposition Kinetics of GD a-Si:H Films", 9th International Conference on Amorphous and Liquid Semiconductors, Grenoble, July 2-8, 1981, in which a-Si:H films were deposited at powers up to 3 watts per square centimeter. Although the concentration of hydrogen and other species in the plasma was monitored by measuring optical emission intensities, it is applicant's understanding that hydrogen gas, as such, was not added to the discharge atmosphere.
Argon has also been used as a diluent in the course of investigating the growth morphology of amorphous silicon films, as described in Knights, "Growth Morphology and Defects in Plasma-Deposited a-Si:H Films", Xerox Palo Alto Research Center, and Knights, et al., "Microstructure of Plasma-Deposited a-Si:H Films", Appl. Phys. Lett. 35 (3), 1979. These articles do not disclose an increase in crystallinity when argon is added, but rather describe an increase in the density of defects in amorphous silicon material. Such defects, taking the form of states in the forbidden gap, hurt the transport properties of a film and are undesirable in most applications. At 230 degrees Celsius, a mimimum defect density was obtained by discharging low rf power through a pure silane atmosphere. The Knights publications thus suggest that the dilution of silane with argon leads to higher surface defects, and is undesirable. In addition, Knights apparently did not add hydrogen gas to his glow discharge atmospheres.
Ogawa, et al., "Preparations of a-Si:H From Higher Silanes (Si.sub.n H.sub.2n+2) With the High Growth Rate", Japanese Journal of Applied Physics, Volume 20, Number 9 (L639-L642) 1981, describes the use of argon as a diluent in a glow discharge of higher silanes. Ogawa attempted to obtain intrinsic amorphous silicon at a higher deposition rate. The article teaches that the deposition rate increases monotonically with an increase in rf power, gas flow rate, and partial pressure of the higher silanes. The Ogawa process does not involve the addition of hydrogen gas to the discharge and, in fact, specifically teaches that hydrogen gas should be eliminated by evacuation after the higher silanes are formed.
Therefore, it is desirable in many applications to provide an improved process for the formation of microcrystalline silicon material at relatively low powers and in relatively short times.